Human Metapneumovirus Subgroup Changes and Seasonality During Epidemics

Background: Human metapneumovirus (HMPV) is a major cause of respiratory tract illness in young children and causes annual outbreaks in winter and spring seasons. We evaluated the subgroups of HMPV that caused annual outbreaks and its seasonal occurrence during a 21-year period. Methods: Real-time PCR was used for detection of HMPV in 3576 nasopharyngeal aspirates that had been continuously collected year-round for the years 1987 to 2008 from infants hospitalized with acute respiratory tract illness. Phylogenetic analysis was used to assess HMPV subgroups. Results: Of the 3576 samples obtained, 202 (5.6%) tested positive for HMPV. All known HMPV subgroups (A1, A2a, A2b, B1, B2) could be identified as important respiratory tract pathogens in infants. We found that one HMPV subgroup predominated each year, and it was displaced by another subgroup every 1 to 3 years. Besides the frequent change in predominant HMPV subgroups, we observed a yearly shift in the seasonal occurrence, with a strong peak of HMPV activity in late spring-summer months every second year. Conclusion: HMPV activity is characterized by a periodic change in the predominant subgroup and it shows a stable seasonal rhythm of alternating winter and spring activity.